Easiet way to install APKs are to load them onto a USB drive and find them with a file explorer
Not limited to mibox. I'm getting the same message on nvidia shield tv and mi box. Hope it's fixed soon.
Maybe Puffin got updated recently and it's not working anymore
Eelq0
have you tried Apitiode TV, the alternative to Google Play.. most apk there are workable in Android TV and set top boxes
https://skyjuiceiswater.blogspot.com/2018/03/the-alternative-to-google-play-for_18.html
Change browser....use another browser, try sideload google chrome apk
I've found the easiest way for me to upload an apk to my Shield is to use X-plore File Manager and Google Drive. Works so easily.